Field vs property vs method

Field vs property vs method#

Classes typically contain data. These data can be read or modified through three ways. Here we introduce them, so you know which to use in which scenario.

Field#

field

a variable in a class

var s1 = new LectureSession
{
    Name = "Industrial Programming",
    StartTime = DateTime.Parse("2025-10-06 13:00"),
    Duration = TimeSpan.FromHours(4),
    StudentCount = 36
};
s1.Duration -= TimeSpan.FromHours(1);
Console.WriteLine(s1.Duration);

public class LectureSession
{
    public TimeSpan Duration;
    public string Name;
    public DateTime StartTime;
    public uint StudentCount;
}

Output:

03:00:00

Property – for regulating access#

Imagine that duration of a LectureSession may not be changed after creating it, because it stays the same whole semester. How can we give read access to a field but forbid writing?

It cannot be private, because otherwise we cannot read this information, however we also cannot leave it as a field, because then we could accidentally modify it. const keyword is also not helpful, because const requires the variable to be known at compile-time, but each lecture session’s duration is determined at runtime when the instance is created.

Properties can help.

property

a special sort of class member, intermediate in functionality between a field and a method.

In the following example Duration becomes a property. Compared to a field, we can control read or write to the variable. For example in the following, Duration can only be written during creation, but cannot be modified.

var s1 = new LectureSession
{
    Name = "Industrial Programming",
    StartTime = DateTime.Parse("2025-10-06 13:00"),
    Duration = TimeSpan.FromHours(4),
    StudentCount = 36
};
s1.Duration -= TimeSpan.FromHours(1);
Console.WriteLine(s1.Duration);

public class LectureSession
{
    public string Name;
    public DateTime StartTime;
    public uint StudentCount;
    public TimeSpan Duration { get; init; }
}

Output:

Scratch.cs(8,1): Error CS8852 : Init-only property or indexer 'LectureSession.Duration' can only be assigned in an object initializer, or on 'this' or 'base' in an instance constructor or an 'init' accessor.

Property with a backing field – for validation#

Using methods we can control access to the variables. For example, a session duration cannot be negative. We can check and throw an error if it is the case as shown in the following example.

var s1 = new LectureSession
{
    Name = "Industrial Programming",
    StartTime = DateTime.Parse("2025-10-06 13:00"),
    Duration = TimeSpan.FromHours(4),
    StudentCount = 36
};
s1.Duration -= TimeSpan.FromHours(1);
Console.WriteLine(s1.Duration);
s1.Duration -= TimeSpan.FromHours(4);

public class LectureSession
{
    private TimeSpan _duration;
    public string Name;
    public DateTime StartTime;
    public uint StudentCount;

    public TimeSpan Duration
    {
        get => _duration;
        set => _duration = value <= TimeSpan.Zero ? throw new ArgumentException("Duration must be positive") : value;
    }
}

Output:

03:00:00
Unhandled exception. System.ArgumentException: Duration must be positive
   at LectureSession.set_Duration(TimeSpan value) in /Scratch.cs:line 22
   at Program.<Main>$(String[] args) in /home/u/projects/ind-prog-code/scratchpad/Scratch.cs:line 10

Activity 55

Augment the last

Method#

method

a procedure (function) associated with an object

Appendix#

[] vs {}#

There is a difference between brackets ([]) and curly braces ({}):

  • () used for methods (including constructors and thus also constructor parameters)

  • {} define a block of code or scope in

    • classes

    • methods

    • loops

    • conditionals

    • object initializers, e.g.,

      var s = LectureSession{
    Name = "Industrial Programming",
    StartTime = DateTime.Parse("2025-10-06 13:00"),
    Duration = TimeSpan.FromHours(4),
    StudentCount = 36
}

So we can use both brackets and braces to initialize data structures.

Record, struct vs classes#

The learning goals of this course can be achieved only with classes, so I left records and structs out.

Record#

Records are syntactic sugar for immutable data classes with the integrated benefits:

  • value-based equality

  • toString()

  • cloning

var s1 = new LectureSession
    ("Industrial Programming", DateTime.Parse("2025-10-06 13:00"), TimeSpan.FromHours(4), 36);
Console.WriteLine(s1);

public record LectureSession(
    string Name,
    DateTime StartTime,
    TimeSpan Duration,
    uint StudentCount
);

Struct#

Structs are class-like containers, but if a struct is assigned, then its values are copied. A class is assigned by reference.

var s1 = new LectureSession
{
    Name = "Industrial Programming",
    StartTime = DateTime.Parse("2025-10-06 13:00"),
    Duration = TimeSpan.FromHours(4),
    StudentCount = 36
};
Console.WriteLine(s1);

public struct LectureSession
{
    public string Name;
    public DateTime StartTime;
    public TimeSpan Duration;
    public uint StudentCount;
}
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